This invention relates to a method for removing an aluminide coating from a metal-based substrate. More particularly, the invention is directed to a method for selectively removing an aluminide coating by using a stripping composition to degrade the coating and then removing it without damaging the substrate. The invention also relates to a turbine engine part having an aluminide coating, at least a portion of which has been selectively removed by the above method.
A variety of coatings are often used to protect metal parts exposed to high temperatures, such as parts made from superalloys. For example, gas turbine engine components (and other industrial parts) are often formed of superalloys that can withstand a variety of extreme operating conditions. Such parts are usually covered with coatings to protect them from environmental degradation, including the adverse effects of corrosion and oxidation. Coatings used on components in gas turbine hot sections, such as blades, nozzles, combustors, turbine shrouds and transition pieces, generally belong to one of two classes: diffusion coatings or overlay coatings.
Diffusion coatings are typically formed of aluminide-type alloys, such as nickel-aluminide; a noble metal-aluminide such as platinum-aluminide; or nickel-platinum-aluminide. Overlay coatings typically have the composition MCrAl(X), where M is an element selected from the group consisting of Ni, Co, Fe, and combinations thereof, and X is an element selected from the group consisting of Y, Ta, Si, Hf, Ti, Zr, B, C, and combinations thereof. Diffusion coatings are formed by depositing constituent components of the coating, and reacting those components with elements from the underlying substrate, to form the coating by high temperature diffusion. In contrast, overlay coatings are generally deposited intact, without reaction with the underlying substrate.
During service, diffusion and overlay coatings are often exposed to oxidative conditions. For example, coatings on turbine airfoils are typically subjected to oxidation in the hot gas path during normal operation. Under such conditions, with temperatures in the range of about 525-1150° C., various oxidative products are formed on the coatings. For example, aluminum oxides and other metal oxides, including nickel oxide, cobalt oxide, chromium oxide, and other base metal oxides, often form on simple aluminide and platinum-aluminide coatings. Aluminum oxides, chromium oxides, and various spinels often form on the MCrAl(X)-type coatings.
When turbine engine components are overhauled, the protective coatings are often removed to allow inspection and repair of the underlying substrate. Various stripping compositions have been used to remove the coatings. Usually, the oxide materials must be removed before the coatings can be treated with the stripping composition. Various techniques have been used for oxide removal. For example, oxide materials often have been removed from external sections of turbine components by grit blasting.
Alternatively, turbine components have sometimes been treated in an oxide-removal solution comprising a strong mineral acid or a strong caustic. Examples of such mineral acids are hydrochloric acid, sulfuric acid, and nitric acid. The caustic solutions usually include sodium hydroxide, potassium hydroxide, or various molten salts. Repeated treatments sometimes are used to remove the oxide. After removal of the oxide is completed, the substrate is then typically immersed in another solution suitable for removing the coating material itself. In current practice, the aluminide materials are often stripped from the substrate by exposure to various acids or combinations of acids, e.g., hydrochloric acid, nitric acid, and phosphoric acid.
There are some drawbacks associated with the use of the various stripping compositions mentioned above. Some stripping compositions do not remove sufficient amounts of the aluminide material. Other compositions that remove the aluminides also attack the base metal of the substrate, pitting the base metal or damaging the metal via intergranular or interdendritic (in the case of single crystal materials) attack. Some stripping compositions are used at elevated temperatures, e.g., above about 75° C. to speed the reaction and removal of the coating. Operation at these temperatures can promote increased attack of the base metal and may require masking materials to protect selected portions of the metal part, e.g., airfoil internal surfaces. Elevated temperature processes also increase energy costs and potentially require additional safety precautions. Airfoil internal surfaces are often filled with wax or plastic to protect surfaces that do not require stripping. These materials must be removed before using the part, adding additional manufacturing steps and cost. Moreover, conventional treatment solutions that employ large quantities of strong mineral acids may emit an excessive amount of hazardous fumes that must be scrubbed from ventilation exhaust systems.
Some processes use grit-blasting prior to acid treatment to pretreat and activate the substrate surface, and after exposure to the stripping composition to remove residual degraded coating. These steps can be very time-consuming, and can also damage the substrate and limit part life. Special care may need to be taken to prevent grit-blasting damage to the substrate or any protective coating not being removed from the metal part. Moreover, grit-blasting cannot generally be used to remove oxide material from internal passages or cavities in metal parts. For example, grit-blasting would not be suitable for use in the internal cooling passages of high pressure turbine blades where the grit particles could block the internal passages.
It is thus apparent that new processes for removing aluminide coatings from metal substrates would be welcome in the art. It would be desirable if the processes remove substantially all of the aluminide coating, while not damaging the base metal. Moreover, it would be desirable if the processes could be carried out at lower temperatures to minimize or eliminate base metal attack. It would also be desirable if the processes eliminate preliminary steps like grit-blasting, so that they can be used to effectively remove coatings from internal sections of metal parts without blocking internal passages.